A typical content delivery network (CDN) may include, among other things, a central server that is the collection point for digital works or “content,” one or more distribution servers, and a collection of receivers. The receivers are typically the end-users of the content distribution network, such as individuals connected via the public Internet and personal Web browsers to the CDN who have paid for or have requested free content transmissions. The distribution servers provide a distributed organization and caching scheme for the content packets. This form of caching is often necessary in large-scale distribution operations in order to provide rapid (low latency) delivery of content packets to the receivers.
The central server may be, in some instances, a single host or a dedicated collection of host servers and storage units responsible for providing CDN management along with the repository of content. Alternatively, the coordination and management functions may be provided by an independent process operating on one or more network host elements. In still other systems, the central server may be merely one of many servers that can all act as content collection points as well as packet distribution points within the network.
It must be noted that “content,” as used in this description, refers to all forms of digital media currently known or to be discovered, whether complete works or portions thereof. Examples include, but are not limited to, videos (e.g., MPEG files), still images (e.g., JPEG or TIFF files), voice and/or musical recordings (e.g., MP3 files), text files (e.g., ASCII or HTML files), Portable Document Format (PDF) files, and executables.
For a variety of purposes, it is sometimes useful to attach tags or markers to the content. These tags or markers are designed so that they are unobtrusive, even invisible to users, and minimally affect the “core use” of the content, but are available for detailed examination by persons or programs that choose to use them. These markers typically contain some kind of tracking information. Well-known examples are the “received” headers of conventional e-mail (described in, for example, Request For Comments [RFC]-821), various kinds of digital watermarks, and region codes on digital video disks (DVDs). (These marks are distinguished from the mechanisms of digital rights management (DRM) systems, which depend on some form of encryption to “lock” the content away as well as some form of key delivery to provide an “unlock” mechanism for the content.)
Tracing or tracking marks support a variety of different applications. One such application is to simply trace the actual path followed by a digital work that might have traversed any of a large number of paths. Another is to identify the point in time at which a work was requested or delivered. A mark can include information that is not easily constructed by another party, offering a weak kind of signature authentication facility. A mark may also provide information that is used by a facility of the recipient to determine which of a number of services might be available. In its crudest form, this latter use determines whether the content is playable or accessible by the recipient's computer.
In another application, marking may be performed by extending fields in previously tagged documents, such as HTML, XML, and related SGML-derived files. Such extensible mark-up languages (or formats) allow the addition of “nested” marks or tags, which are simply ignored by readers that are not looking for them.
In addition, other forms of watermarking or postmarking are also well known. Such uses include, for example, date- and time-stamps, copyright information, and even steganometric encryption of additional information within an image. Such devices are described, for example, in Bruce W. Schneier, Applied Cryptography, 2d ed. (1996) and J. Zhao, “A WWW Service To Embed And Prove Digital Copyright Watermarks,” Proc. of the European Conference on Multimedia Applications, Services and Techniques, Louvain-La-Neuve, Belgium (May 1996), both incorporated herein by reference in their entireties.